1. Single rate plateau curve with different gas mixture
Standard cell prototype with SLD gas mixture
If we want a plateau region starting at lower voltage we need to increase Ar in our gas mixture. The plateau with Ar 16%, Isob 9% and CO2 75% has a plateau curve very similar to the one found for the 9x9 standard cell prototype with SLD gas mixture

2. I choose Ar/Isob/CO2 – 16/9/75
Rates are normalized by the number of HV channel.
The plateau region is acceptable: ~ between 4.8KV an 5.2KV.
All the tube has very similar behavior.
Tube number 2 still has a cell which sparks frequently: this tube should be rejected.

3. Gas mixture: Ar/Isob/CO2 – 16/9/75 @ 5000V
Signals from the scope
Ch. 3 (yellow) phi strip signal (no amplified) – Ch. 4 (blue) wire OR signal (threshold 30 mV)
Gas mixture: Ar/Isob/CO2 – 16/9/75 @ 5000V